1. Field of the Invention
The present invention generally relates to cutting apparatus for an optical disc and more particularly to a cutting apparatus for an optical disc in which a master disc of a video disc is provided by using a laser light source.
2. Description of the Prior Art
Video and audio signals are recorded on a prior-art video disc as shown by a frequency allocation of FIG. 1.
As shown in FIG. 1, a video signal and a 2-channel audio signal are modulated to provide FM signals by FM modulators (not shown). More specifically, the video signal is FM-modulated to provide a video FM carrier signal 30 having a central frequency of 8.5 MHz (frequency deviation is 1.7 MHz), and the stereo 2-channel audio signal is FM-modulated to provide a first channel audio FM carrier signal having a central frequency of 2.3 MHz and a second channel audio FM signal (referred to hereinafter as an analog audio signal) 31 having a central frequency of 2.8 MHz (frequency deviation is in a range of .+-.100 kHz). Further, the 2-channel audio signal is pulse code modulated (PCM-modulated) by 16 bits to provide an eight-to-fourteen modulation (EFM)-coded digital audio signal 33. This digital audio signal 33 is inserted into the low band region lower than 2 MHz as shown in FIG. 1.
FIG. 2 shows a systematic block diagram of a prior-art cutting apparatus in which the above-mentioned video and audio signals are recorded on a photoresist coated on a glass master disc.
Referring to FIG. 2, a video signal to be recorded is recorded by a video tape recorder (i.e., VTR) 35 and the audio signal is recorded by a tape recorder 36 or the like in a similar fashion. The video signal is supplied from the VTR 35 through a switch 28 to a video modulator 25 provided within a video signal processing circuit, in which it is FM-modulated. The first 2-channel audio signal is also supplied from the tape recorder 36 through a switch 29 to an audio modulator 26 provided within an audio processing circuit, thereby being FM-modulated. The video FM-modulated wave from the video modulator 25 and the audio FM-modulated wave from the audio modulator 26 are supplied to a mixing circuit 24.
A second audio signal is recorded by a tape recorder 37 and is supplied through a switch 34 to an analog-to-digital (A/D) converter (not shown), in which it is converted into a digial audio signal. This digital audio signal is pulse code modulated (PCM-modulated) by an EFM encoder 27, filtered out in its band higher than 2 MHz by a low-pass filter (not shown), pre-emphasized in its low band by a pre-emphasizing circuit (not shown) and is mixed with the video signal and the first audio signal by the mixing circuit 24.
An output side of the mixing circuit 24 is connected to a non-inverting input terminal of an operational amplifier 22 whose inverting input terminal is connected to one end of a variable resistor 23. The other end of the variable resistor 23 is grounded, and an offset adjustment is performed by the operational amplifier 22 and the variable resistor 23. An output of the operational amplifier 22 is supplied to a limiter 21, from which it is further supplied to an acousto-optic modulator (referred to hereinafter as an AOM) driver 19 and a spectrum analyzer 20. That is, the video FM carrier signal is supplied to and observed by the spectrum analyzer 20 and the variable resistor 23 is adjusted in a manual fashion so that a secondary higher harmonic component of the main carrier may be minimized. The final output from the limiter 21 is supplied to an acousto-optic modulator (AOM) 8 as a square wave.
The AOM 8 is formed such that a voltage of frequency f is applied to a piezoelectric material (e.g., LiNbO.sub.3) to produce a compression wave having a sonic velocity v and having a wavelength .LAMBDA. in a medium (e.g., Te glass). If this medium is used as a diffraction grating and a laser beam having a wavelength .lambda. is acted on the diffraction grating, then the laser beam causes Bragg scattering or Bragg reflection. This reflected and diffracted beam is changed into a voltage which drives the medium so that, if this voltage is amplitude-modulated, it is possible to obtain a modulated laser beam. For this reason, the AOM 8 is interposed in the light path of a laser beam emitted from a laser light source 1 such as an argon laser, a helium-cadmium laser or the like. A laser beam emitted from the laser light source 1 is supplied to an electro-optic modulator (referred to hereinafter as an EOM) 2. This EOM 2 is made of a uniaxial crystal such as KH.sub.2 PO.sub.4 (potassium dideuterium phosphate, i.e., KDP), NH.sub.4 H.sub.2 PO.sub.4 (ammonium dihydrogen phosphate, i.e., ADP) or the like. It is to be appreciated that, if an electric field is applied to this uniaxial crystal, then a difference of phase velocity proportional to electric field intensity occurs between two planes of linearly polarized waves advancing within the crystal. Therefore, if a linearly polarized light is introduced into this uniaxial crystal, such linearly polarized light becomes an elliptic polarized wave corresponding to the applied electric field. For this reason, if a spectrum analyzer 3 is provided behind this crystal or EOM 2, it is possible to obtain the amplitude-modulated laser beam. The thus amplitude-modulated laser beam is introduced into a first beam splitter (i.e., BS.sub.1) 4. A first photodetector (referred to as a PD.sub.1) 5 such as a photo-diode is located at the rear stage of the first beam splitter 4. The first photodetector 5 receives a laser beam derived from the first beam splitter 4, converts the same into an electrical signal and controls the electric field of the EOM 2 in association with an automatic power control (i.e., APC) circuit 6 such that the amount of laser beam passing through the first photodetector 5 may become constant. The laser beam, reflected by the first beam splitter 4, is supplied to the AOM 8 interposed between lenses 7 and 9a, in which the information signal is modulated, and introduced through an aperture 9b into a second beam splitter (i.e., BS.sub.2) 10.
A laser beam, reflected by the second beam splitter 10, travels through a lens 11, a .lambda./4 wavelength plate 12, a mirror 13 and an objective lens 14, in that order and exposes a photoresist 16 coated on a glass master disc 15. When this exposed photoresist 16 is developed, concave and convex pits are formed, thus the master disc being constructed. Then, on the basis of this master disc, a stamper is constructed according to the nickel plating process or the electroforming process, and a disc substrate of video disc is copied by the injection molding-process by using this stamper.
The glass master disc 15 is held on and rotated by a turntable 17 which is revolved by a drive motor 18. In that case, the glass master disc 15 can be moved in the width direction thereof by a feeding mechanism (not shown).
In the prior-art cutting apparatus as described above, the automatic power control (i.e., APC) circuit 6 is controlled so that, in the cutting-process, the cutting laser beam emitted from the laser light source 1 may have a constant power corresponding to a linear velocity for a CLV (constant linear velocity) disc or that the cutting laser beam may increase its power in the outer periphery of the optical disc for a CAV (constant angular velocity) disc.
That is, the laser beam emitted from the laser light source 1 is adjusted in offset before being modulated by the ON-OFF control of the AOM 8. Because of this, when the laser beam emitted from the laser light source 1 travels through the AOM 8, this laser beam is changed very slightly due to temperature characteristic, aging change, environment factors or setting conditions with the result that the pits are not formed uniformly on the master disc 15.
In the prior-art cutting apparatus, as shown in FIG. 2, the square wave mixed by the mixing circuit 24 and amplitude-limited by the limiter 21, i.e., the repetitive frequency of the square wave in which the video FM wave signal is pulse width modulated (PWM-modulated) by the audio FM wave signal represents the video signal. From a pitch train standpoint, the higher the frequency of the video FM modulated signal becomes, the shorter the pit length becomes and the space between the pits is reduced more. The duty ratio, W/T, that is, the ratio of square wave pulse width W relative to one cycle time T of square wave, i.e., "ON"-"OFF" ratio is adjusted such that, while the final output of the limiter 21 is observed by the spectrum analyzer 20, the secondary higher harmonic component of the main carrier is minimized. Even though the output of the AOM 8 is optimized, the duty ratio of the "ON"-"OFF" modulated laser beam is not always optimized due to non-linear characteristic of the AOM 8 or the like, thus making the pit duty incorrect.